From Fe(III) Ions to Ferrihydrite and then to Hematite

The pathways from soluble Fe(III)-aquo ions to various solid polymeric Fe(III) oxides of increasing thermodynamic stability involve intermediate products which are still only partly known. This paper combines results from the literature with own new results from X-ray diffraction, Mössbauer spectroscopy, and electron microscopy. It is maintained that no intermediate phases were positively identified between mono-, di-, and trimers and a range of solid polynuclear phases. This indicates fast polymerisation as the OH/Fe ratio of the system increases. The immediate solid polynuclear phases are poorly crystalline Fe(III)-oxyhydroxy salts and a range of oxyhydroxides, called ferrihydrites in mineralogy, with varying crystallinity and magnetic ordering behavior. The rate of hydrolysis as affected by pH, rate of OH addition and temperature is of paramount importance for the nature of these polymeric phases. In the presence of free or surface-bound water, the transformation of ferrihydrite into the stable end product hematite (alpha-Fe2O3) proceeds by crystallization within the ferrihydrite aggregate and is enhanced as the pH approaches the zero point of charge as well as by Al in the system. This mechanism is different from that in nonaqueous systems. Copyright 1999 Academic Press.